Vitamin D A mother's vitamin D levels during pregnancy has no affect on her children's bone health, a UK study has found, although the findings have been met with a mixed response.

Professor Debbie Lawlor of the University of Bristol, and team, report their findings in today's issue of The Lancet.

Lawlor and colleagues examined data on vitamin D levels of nearly 4000 pregnant women at different stages of their pregnancy.

Of the 2049 women monitored in their third trimester, 26 per cent were deemed to have insufficent levels of vitamin D, and 6 per cent were deficient.

The researchers then looked at the bone mineral content (BMC) of the women's children, just before they turned 10. BMC is a measure of bone health and a low BMC is associated with diseases such as rickets.

"We found not relevant association between maternal vitamin D status in pregnancy and offspring BMC in late childhood," write Lawlor and team.

"No associations with offspring BMC were found for any trimester, including the third trimester, which is thought to be most relevant

The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) say the study is well done.

Robson says there is a lot of uncertainty over how best to deal with vitamin D deficiency, including how often to screen for it and how best to treat it.

He says the study addresses one particular worry that women avoiding the sun could become vitamin D deficient and this could affect their offspring.

"There was all this concern that we were going to have a new generation of kids with rickets, but that doesn't seem to be the case," says Robson.

"I think it's very reassuring."

An expert in the public health aspects of vitamin D at the University of Melbourne, Professor Peter Ebeling agrees.

"It's by far the best [study] that's been done so far," he says. "It's reassuring ... that we don't really need to treat every pregnant woman with vitamin D to avoid their children having low bone mass."

Ebeling says up to 42 per cent of pregnant women are deficient in vitamin D levels [defined by serum levels below 50 nanomoles per litre] and those at highest risk are those with pigmented skin, who cover their skin, or who avoid the sun.

Study limitations

But Professor John Wark, also from the University of Melbourne, who has been involved in studying the impact of vitamin D levels in pregnancy on offspring urges caution about the study's findings.

He says previous studies on the link between maternal vitamin D and child bone health were inadequate for various reasons, including their small size.

But, while the size of the latest study is "impressive" Wark believes it has a number of important limitations.

For example, he says, the study appears to be an "opportunistic" rather than pre-planned prospective study. Women were included in the study if they had had vitamin D levels measured, but there is no information on why they had these levels measured.

"There could be all sorts of reasons why they had vitamin D levels measured during antenatal care if it wasn't routine," says Wark. "That introduces the possibility of all sorts of biases both known and unknown."

Also, he says, it appears the study did not control for the possibility that women with low vitamin D levels subsequently took vitamin D supplements, which would also confound the results.

"I think the limitations of this study are such that the study doesn't contribute to informing clinical practice at the present time," he says. "I think the whole field desperately needs a well-conducted randomised controlled trial."